Elevators conventionally have a car and a counterweight with these being connected by cables passing over a support sheave. The counterweight is selected of a weight between the empty and fully loaded weight of the car. Normal braking is accomplished by controlling the drive motor speed and torque to bring the car to a complete stop at the floor. Once at the floor, power is removed from the drive motor and a spring loaded friction brake is used to hold the car at the floor.
A safety is located on the car frame which engages the guide rails on downward overspeed of the car. Such engagement of the rails is not desirable in the upward direction because of the possibility of stopping with a greater than 1-G deceleration if the safety jams.
It is also known to prevent energization of the drive motor when the doors are open and the car is greater than a preselected distance from a landing. Some discrete movement is desirable to permit leveling of the elevator car, provided that the car is within close proximity of the landing with the doors open.
It is also possible, however, to experience an upward overspeed of the car. For instance this can occur with malfunction of the brake or control system and a lightly loaded car. This is particularly a problem when the car is at a low elevation so that substantial speed can be obtained by the time the car reaches the overhead building structure.
Movement from the floor can possibly occur even with the drive motor deenergized. Therefore, it is desirable to have a safety braking action to stop movement of the car beyond a predetermined distance with the doors open.
Certain elevator code regulations are in progress requiring a safety on upward overspeed and also on movement up or down beyond a specified distance with the elevator doors open. These codes will normally require that the safety braking system be independent of the regular controls.
Tripping of the conventional rail safety causes damage to the guide rails requiring rework. It would be convenient to have a downward overspeed safety operable before the rail safety operates, with the alternate safety either requiring no rework, less rework than the conventional guiderail safety, or inexpensive replaceable parts in the event of damage.